1. Field of The Invention
This invention relates to aqueous dispersions of phenolic resin. In particular, this invention relates to stable resin dispersions that are heat-reactive, have outstanding chemical resistance properties, and are safe for use in contact with foodstuffs.
2. Description of Related Art
Phenolic resins are commercially important materials that are used in an array of diverse products. Examples include coatings, binders for inorganic materials, binders for organic materials, rubbers, laminates, moldings, castings, and other uses. Phenolic resin polymers are also widely used as a component in coating materials that are baked to form the coating and to adhere the coating to the substrate. Resins that tend to exhibit hydrophobicity typically provide less resistance to chemical attack. Whereas such hydrophobic polymers can be used to form aqueous heat-reactive coating compositions, they typically are not considered suitable for use in many coating applications because of their limited resistance to chemical attack.
Historically, phenolic resins were only available in organic solvent based formulations. Such formulations have lately come under pressure as manufacturers seek to find formulations that reduce the amount of volatile components. Governmental regulations often mandate significant reduction, or even elimination, of volatile organic solvents from such formulations, particularly for products having selected uses. The reduction or elimination of volatile organic solvent emissions has provided new commercial opportunities for resins dispersions with low levels of volatile organic compounds.
The art has investigated a variety of ways to make a useful, stable aqueous phenolic resin dispersion that do not pose a significant loss in either handling or performance characteristics. Aqueous dispersions have been made with gums or cellulosic materials as the dispersing agent. Such gum and cellulose-containing dispersions were, however, unsatisfactory. The storage stability was poor, and the coating suffered from poor chemical resistance due to the hydrophilic nature of the dispersing agent.
Surfactants, such as phosphate esters, have been tried. Formulations containing such surfactants provide unacceptably poor film properties caused by surface migration of the surfactant.
The use of a protective colloid material, particularly PVA, as a dispersant has been the subject of attention in the art. Such protective colloids have been used with polyacrylamide and an optional aldehyde-reactive modifier in U.S. Pat. No. 4,131,582. A protective colloid is used to stabilize a phenolic resin formed in situ with other components of the adhesive formulation according to U.S. Pat. No. 4,400,229. Novolac resins are stabilized with PVA in U.S. Pat. No. 4,788,236. Protective colloids can also be used in systems relying on an organic coupling solvent for the phenolic resin. See, U.S. Pat. Nos. 4,124,554 and 5,200,455. Unfortunately, PVA resin works well only with hydrophobic phenolic polymers (e.g., bisphenol-A) and does not form a stable dispersion with highly functional, hydrophillic resins. It would be useful to have an aqueous dispersion of such hydrophillic resins because such resins are characterized by better chemical resistance than the hydrophobic bisphenol-A resins.
There exists in the art the need for an aqueous phenolic resin formulation that provides a coating material which exhibits storage stability, chemical resistance, and ease of handling.